From Halopedia, the Halo wiki
This article is about the achievement. For the MJOLNIR variant, see VANGUARD-class Mjolnir.
Vanguard is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by beating the par score on the level The Covenant. The achievement is worth 10 Gamerscore. It is represented by a blue circle with a coiling dragon, the symbol for a canceled Bungie product, Phoenix, in the original game and a screenshot of Last Resort in the collection.
The par score for The Covenant is 50,000 points in Halo 3 and 25,000 points in The Master Chief Collection; the difference is due to the fact that the two games feature different campaign scoring formulae. Though the achievement can theoretically be unlocked on any difficulty, more points are awarded in campaign scoring when playing on higher difficulties.
In the original game, players must achieve the par score individually, even when playing cooperatively; in The Master Chief Collection cooperative players share a single score, potentially making the achievement much easier.